1. Field of Invention
The present invention relates to an induction heating device utilizing an open core of ferrite material provided with a coil of Litz wire in which passes an excitation current to produce a variable magnetic field which is concentrated in a high flux density between the poles of the open core by means of magnetic flux concentrator which are made of electrically conductive tube is close contact with a thermally conductive and electrically non conductive material to drain the heat generated in the coil and in the core, wherein a cooling fluid is circulated through the concentrator tube.
2. Description of Prior Art
A variety of types of high frequency induction heating devices have been proposed in the prior art. U.S. Pat. No. 4,359,620 provides a good summary of the prior art where it is described that one of the problems encountered with many induction heaters, utilizing magnetic cores, is that of high heat losses in their core. This is particularly true if the field intensity and frequency of the fluctuating magnetic field generated is increased sufficiently to be adequate to, for example, solder metal. However, this causes the problem of increasing the temperature of the core, and the core begins to melt. Cores made of laminated magnetic materials used in most of transformers have very high losses due to both eddy currents and to the resulting skin effect at frequencies above 20 Kc. Also, the conductive nature of core laminates present a real danger of electrical shock when used in induction heaters which have a large amount of power supplied to their exciting coils.
In attempt to diminish this problem, U.S. Pat. No. 2,785,263 discloses the use of cores made of ferrite. Such material has relatively high magnetic permeability and low conductivity and has been found to be an ideal material for use in induction heaters. However, other problems have resulted by the use of such cores and namely that in order to saturate the pole pieces so that they can contribute to the maximum to the flux density generated in a work piece placed between them, it is necessary to sature substantially to whole core, and this is very inefficient and at high frequencies result in huge heat losses. U.S. Pat. No. 4,359,620 attempts to solve this further problem by utilizing a core design which focuses a magnetic field of high flux density between its two ends which are closely spaced and tapered. A periodic voltage is supplied to the coil and a capacitance is connected with the exciting coil to form a resonance circuit which is used to control the frequency and phase of the periodic voltage supplied to the resonance circuit to maintain it in resonance. Again, this patent does not deal with the high heat losses in the core and the problem of the core and the coil being subjected to high temperatures which places a restraint on the magnitude of the intensity of the flux density of the magnetic field generated, thereby limiting the application of the induction heater due to its poor heat resistance and lack of uniform heating.